ORDER ACTINOIDEA. 



75 



the disk and cavity may continuously enlarge ; in the latter case, the 

 buds open in the disks, the process of budding being the cause of 

 their enlargement. 



b. The following figure of a foliaceous Echinopora illustrates the 

 process where the growth is a simple marginal prolongation not ex- 

 tending to the disks. 



Fig. 33. 





Echinopora. 



By the prolate mode of growth, the polyps gradually extend out- 

 ward, and new buds open, from time to time, a short distance from 

 the edge, and have no connexion at base with the preceding polyps. 

 The corals of these species have the upper surface striated, indicating 

 the presence of visceral lamellae within the extending part, from 

 which buds may proceed. The contrast between this and the Gem- 

 mipora ($ 66), in each of the above particulars, is made apparent by 

 a simple comparison of the figures; for in the latter, the buds, besides 

 opening at the very margin, may be traced within to a preceding 

 polyp, from which each is a lateral shoot: and, moreover, the surface 

 is not striated. 



c. The above Echinopora, and other foliaceous corals among the 

 Astraidse, are examples of prolate growth without the acrogenous. 

 But the massive Astrseas are produced by the united action of these 

 two modes of growth, and their hemispherical 

 forms result from the perfect regularity and 

 symmetry in the process of budding. 



d. Several species of Astrreas afford examples 

 of the mode of gemmation, illustrated in the 

 Echinopora, among which is the Astrsea argus. 

 While the margin of the hemisphere is extend- 

 ing by the multiplication of buds, precisely 

 similar to that above, the whole surface is also gra- 

 dually enlarging by a widening of the intervals 

 between the polyp-disks. But this widening is kept within limits by 

 the appearance of new buds in these intervals when they begin to exceed 



Fig.-34. 



Astrea argus. 



